Is it possible to place plywood onto slatted base to stop dipping

Hi there
I've had two mattresses now which dip in the middle (just got a new one and while it was okay at first its started to dip within a week). I'm guessing from your site this is because of the sprung slat system I have. Is it possible to just get a piece of plywood to stick on top of the slats and should these fit the size of the bed frame?
Thanks
Aaron